//Retrieve user's last login date
$u = new User();
$ui = UserInfo::getByID($u->getUserID());
$lastLogin = $ui->getLastLogin(); //<--if this doesn't work, try $ui->getPreviousLogin();
//Retrieve parent page path
$parentCID = 81;
$parentPage = Page::getByID($parentCID);
$parentPath = $parentPage->getCollectionPath();
//Retrieve pages under parent page that have been added since user's last login
Loader::model('page_list');
$pl = new PageList
$pl->filterByPath($parentPath);
$pl->filterByPublicDate(date('Y-m-d', $lastLogin), '>=');
//$pages = $pl->get(); //<--uncomment this if you want the pages themselves (not just the count)
$pageCount = $pl->getTotal();